The present invention relates to a radio communication system in which a plurality of radio communication stations exchange data. More particularly, the invention relates to a radio communication system in which, when radio stations of relatively small output powers exchange data, data transmission is relayed to extend the communication area.
When a plurality of radio stations having no master station exchange data, a radio station as a transmitter specifies a destination radio station when transmitting data. However, in radio equipment which needs no license, such as specific low-power radio equipment or minute power radio equipment, since the output power from the radio equipment is low, a relay station is required to extend the transmission range.
In the conventional radio communication system, the 2-frequency simplex relay method or the 4-frequency duplex relay method is generally employed, as disclosed in xe2x80x9c400 MHz band data transmitting radio equipment for a specific low-power radio station (RCR STD-17A)xe2x80x9d.
FIG. 1 shows a typical four-frequency duplex relay system.
Communication between a radio station A(1) and a relay station R1(2) and communication between a relay station R2(3) and each of radio stations B1(4), B2(5), . . . Bn(6) are performed using pair frequencies of different frequency channels, respectively. In this relay system, when the radio station A(1) and the radio station Bn(6) communicate through the relay stations R1(2) and R2(3), the calling radio station A(1) is connected with the relay station R1(2) according to a line connection procedure. In FIG. 1, a frequency Fa(7) at which the station A(1) communicates with the relay station R1(2) and a frequency Fr1(8) at which the relay station R1(2) communicates with the station A, are pair frequencies of a predetermined frequency channel. A signal received by the relay station R1(2) is once demodulated to a base band signal, transmitted to the relay station R2(3) as the base band signal, and then transmitted from the relay station R2(3) to a radio station which is ready for receiving, through an appropriate line connection procedure. The radio station Bn(6), which has recognized that the transmitted signal is data directed to the self-station, acknowledges by using one of the pair frequencies of the channel used by the relay station R2(3). Thereby, the connected radio stations A(1)and Bn(6) can exchange data.
As an alternative, there is a relay system shown in FIG. 2 in which each of plural radio stations is provided with a relay function, and these radio stations are interconnected to relay data in cooperation with each other. In this system, when a radio station A(10) exchanges data with a radio station E(14), initially, the calling radio station A(10) outputs a connection request directly to the radio station E(14). However, since the radio station E(14) is located outside an area 15 within which data from the station A(10) is reachable (hereinafter referred to as xe2x80x9cstation A areaxe2x80x9d), the radio station E(14) cannot receive the request and, therefore, it cannot acknowledge the request.
Next, the radio station A(10) outputs relay requests simultaneously to many unspecified radio stations which are located within the station A area 15. Then, radio stations B(11), C(12), and D(13), which have received the relay requests from the radio station A(10), acknowledge that they are ready for relay, while confirming an absence of signals transmitted from other stations in a carrier sense. Since the radio station A(10) can recognize the communicable radio stations by the ID codes of the radio stations which have made the acknowledgments, it inquires to each radio station whether it can be connected to the radio station E(14) or not. On receipt of the inquiry, each radio station outputs a connection request to the radio station E(14). In FIG. 2, the radio stations C(12) and D(13) are connectable to the radio station E(14), and the radio station A(10) which has received this information outputs a relay request to one of the radio stations C(12) and D(13) to perform data exchange with the radio station E(14) through this relay station.
In the conventional relay system shown in FIG. 1, since the relay stations must be arranged in advance as radio equipment, the data transmissible range is decided by the arrangement of the relay stations. Therefore, when the calling station or the called station is outside the communication ranges of the relay stations, these radio stations cannot exchange data.
Further, in the radio communication system shown in FIG. 2 wherein each radio station is provided with a relay function, since each radio station can perform the relay operation independently, the communication range is extended as compared with the relay system where the relay stations are fixed. In this system, however, it is necessary to search all of the radio stations which can communicate with the calling station for an object relay station until the calling radio station is connected to the called station, and this complicates the line connection procedure by relay and increases the time required until the line connection is completed.
The present invention is made to solve the above-described problems and has for its object to provide a low-power radio communication system for relaying transmitted data to extend the communication area, in which, when a calling station is connected with a called station via a relay station, an optimum communication path can be selected without requiring many complicated procedures.
Other objects and advantages of the invention will become apparent from the detailed description that follows. The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the scope of the invention will be apparent to those of skill in the art from the detailed description.
According to a first aspect of the present invention, there is provided a radio communication system in which a plurality of radio stations perform data exchange by radio communication. In this system, each of the radio stations has a function of outputting data to another radio station in the radio communication system, a function of receiving data from another radio station, and a function of relaying data in response to a request from another radio station. When the radio station outputs data to a specific target radio station but there is no acknowledgment from the target radio station, the radio station outputs relay requests to many unspecified radio stations, and the radio stations, which have received the relay requests, relay and output the data simultaneously to the target radio station. Therefore, the radio terminal can complete the line connection request to the target radio terminal by the minimum procedure, whereby a number of complicated procedures and time required for line connection can be reduced.
According to a second aspect of the present invention, in the above-described radio communication system, when the target radio station, which has received the data relayed by the many unspecified relay stations, acknowledges the receipt of the data, it selects an optimum radio station for relaying the data from the reception level status of radio packets which have been simultaneously transmitted from the many unspecified radio stations, and outputs acknowledgment data to the selected radio station. Therefore, the target radio station can select an optimum relay station from plural relay stations, whereby a communication path via a relay station can be constructed by the minimum line connection request.